[Haskell-cafe] Function signatures and type class constraints

[Oscar Finnsson]

Thanks for the tip. You saved my day (and code)!

So what is the point of having the constraint on the left side of the
'='? Will it allow me to do anything that the right-side constraint
won't?

-- Oscar

On Mon, Aug 23, 2010 at 11:06 PM, Daniel Fischer
<daniel.is.fischer at web.de> wrote:
> On Monday 23 August 2010 22:30:03, Oscar Finnsson wrote:
>> Hi,
>>
>> I'm wondering why I have to repeat the class constraints at every
>> function.
>>
>> If I got the data type
>>
>> > data (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data
>> > c) => Foo a b c = Foo a b c
>>
>
> Type class constraints on datatypes are considered a wart. They don't do
> what people expect, in particular they don't make the constraints available
> at the use site.
>
> It works if you move the constraints across the '=':
>
> {-# LANGUAGE ExistentialQuantification #-}
>
> data Foo a b c = (Eq a, Show a, ...) => Foo a b c
>
> or with GADT syntax:
>
> {-# LANGUAGE GADTs #-}
>
> data Foo x y z where
>  Foo :: (Eq a, Show a, ...) => a -> b -> c -> Foo a b c
>
> Both make the constraints available at the use site,
>
> bar :: Foo a b c -> String
> bar (Foo a b c)
>    = "Foo " ++ show a ++ " " ++ show b ++ " " ++ show c ++ ", Yay!"
>
>> and then a function from Foo to String I have to supply the signature
>>
>> > bar :: (Eq a, Show a, Data a, Eq b, Show b, Data b, Eq c, Show c, Data
>> > c) => Foo a b c -> String
>>
>> even though it should be clear that a, b and c *must* fulfill the
>> constraints already so I should be able to just supply the signature
>
> One would think so. It's a wart.
>
>>
>> > bar :: Foo a b c -> String
>>
>